The CXC chemokine receptor 4 (CXCR4) is a G protein-coupled receptor (GPCR) with stromal cell-derived factor-1 (SDF-1 or CXCL12) as sole published ligand. CXCR4 is involved in multiple developmental and physiological processes including stem cell homing (Möhle and Drost, 2012) and migration of immune cells (Campbell et al., 2003). The CXCR4-CXCL12 axis also plays a role in innate and adaptive immunity, as well as in various disease processes, such as cancer cell metastasis, leukemia cell migration, rheumatoid arthritis and pulmonary fibrosis (Nagasawa et al., 1996; Zou et al., 1998; Tachibana et al. 1998; Furze et al., 2008). Man-made CXCR4 antagonists are capable of mobilizing hematopoietic stems cells (HSCs), which are utilized for immune reconstitution after organ transplantation or chemotherapy (Ratajczak and Kim, 2012; Schroeder and DiPersio, 2012). In addition, CXCR4 is also a major co-receptor for HIV-1 entry into target cells (Feng et al., 1996; Bleul et al., 1996). Co-receptor utilization of CXCR4 is highly effective and a high proportion of CD4+ T cells express this GPCR in lymphatic tissues in vivo. Nonetheless, almost exclusively HIV-1 variants utilizing the C—C chemokine receptor type 5 (CCR5) are transmitted and found during chronic HIV-1 infection (Alkhatib et al., 1996; Deng et al., 1996; Dragic et al., 1996). It has been proposed that multiple factors contribute to the inefficient transmission of CXCR4-tropic (X4) HIV-1 strains (Margolis and Shattock, 2006). However, the mechanism(s) underlying the effective control of X4 HIV-1 in immunocompetent individuals remain poorly understood.
Research on CXCR4-antagonists has recently become an immense field of projects due to the manifold indications In particular the efforts to find a strategy to intervene with cancer cell proliferation, differentiation, and metastasis was not so successful in clinical studies yet as expected. The development of one of the compound groups, namely AMD3100 a CXCR4-antagonists
(a bicyclame compound: Hendrix and Flexner 2000), had to be stopped for long term treatments due to toxic side effects. Although AMD is registered for single short applications in stem cell mobilisation, it is nevertheless a challenge to find adequate antagonists to the target CXCR4.